The invention relates to a magnetic resonance apparatus, comprising a magnet system for generating a steady magnetic field, a gradient magnet system, and an rf coil system which comprises an rf coil in which mutually parallel current paths extending axially across a cylindrical surface can be introduced, and also relates to a method of designing an rf coil for such an apparatus.
A magnetic resonance apparatuses of this kind is known from EP 213 665 which corresponds to U.S. Pat. No. 4,737,718. The apparatus described therein comprises a bird-cage rf coil which is composed of two ring conductors which are interconnected by way of a number of axial extending longitudinal wires which extend parallel to one another and parallel to a symmetry axis and which are situated on a cylinder defined by the ring conductors. When impedances and reactances are suitably chosen, a spatially uniform rf field can be generated in the coil by means of a cosine wave applied to the ring conductors.
U.S. Pat. No. 4,712,067 describes an rf coil which is composed of a number (for example two) of saddle-shaped coils whereby, using a suitable mutual coupling and correct control, a spatially uniform rf field can be generated in a mode which depends on the control and the mutual orientation of the current conductors. Current conductors which extend mutually parallel and parallel to a symmetry axis again form active longitudinal wires of the coil which are situated on a cylindrical surface.
EP 218 290 which corresponds to U.S. Pat. No. 4,839,595 describes an rf coil in the form of a so-called surface coil. Such a coil comprises a current conductor which is situated in one plane and through longitudinal wires of which there extends an imaginary cylinder, on a cylindrical surface of which the longitudinal wires extend in the axial direction so as to be mutually parallel and parallel to a symmetry axis. A plurality of such coils can be combined as described so as to form a butterfly coil.
In the types of coil described therein an rf field having a reasonably spatial homogeneity can be generated in a space which is enclosed by the current conductor or which is defined thereby and spin resonance signals can be detected from such space. The dimension of the rf measuring field space is determined by the geometry of the coil and will always be smaller than a space defined by the geometry of the coil. The filling factor which is to understood to mean herein, the part of the space in which the rf field is sufficiently uniform and which forms part of the geometrically defined space, can be increased in the radial direction by using a larger number of longitudinal conductors. Inter alia because a larger radius of a cylindrical coil makes the coil space more accessible, an acceptable solution can be found, at the expense of a higher energy consumption. In the axial direction the filling factor is reduced by increasing the diameter while maintaining the axial length of the coil; a smaller diameter makes the coil less accessible. When the length of the coil is increase, the rf measuring field space becomes larger but more energy will be consumed and the patient load will increased at areas where the load is not effectively used.
Homogeneity of rf coils for MR apparatus is important both for uniform excitation of spins and for uniform reception sensitivity for spin signals.
In addition, for transmitter coils it is important, from a point of view of rf power supply, and for receiver coils it is important from a point of view of signal-to-noise ratio that the rf coil has a sensitivity which is as compatible as possible with the space in which MR takes place. This implies a high filling factor. Known coils all exhibit a substantial dispersion field in the direction of the main axis. The ideal field profile of an rf coil is a block: sensitive only in the center and insensitive outside the center.
Because coils have a finite length in practice and are not bounded by an ideally conductive shield, the amplitude will always decrease towards the edge and the field will project outside the geometrical coil volume.
The transverse homogeneity is dependent on the number of current conductors situated on the circumference of the coil. In contemporary coils the current intensity in the conductors is cosinusoidal modulated as a function of the circumferential angle. The current intensity in a conductor, however, is constant. Thus, the homogeneity in non-central transverse planes is poor and the profile of the field along the main axis necessitates a long coil which, however, will have a poor filling factor. Homogeneity and field definition are in principle incompatible and are even contradictory in conventional coils.